1. Field of the Invention
The present invention relates generally to electrodes for use in batteries. More particularly, this invention relates to methods of forming alkali metal electrodes having a reinforced glassy protective layers.
2. Description of Related Art
In theory, some alkali metal electrodes could provide very high energy density batteries. The low equivalent weight of lithium renders it particularly attractive as a battery electrode component. Lithium provides greater energy per volume than the traditional battery standards, nickel and cadmium. Unfortunately, no rechargeable lithium metal batteries have yet succeeded in the market place.
The failure of rechargeable lithium metal batteries is largely due to cell cycling problems. On repeated charge and discharge cycles, lithium “dendrites” gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow “mossy” deposits which can dislodge from the negative electrode and thereby reduce the battery's capacity.
To address lithium's poor cycling behavior in liquid electrolyte systems, some researchers have proposed coating the electrolyte facing side of the lithium negative electrode with a “protective layer.” Such protective layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Many techniques for applying protective layers have not succeeded.
Some contemplated lithium metal protective layers are formed in situ by reaction between lithium metal and compounds in the cell's electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode.
Various pre-formed lithium protective barrier layers have been contemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes an ex situ technique for fabricating a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (“LiPON”) or related material. LiPON is a glassy single ion conductor (conducts lithium ion) which has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.).
One difficulty encountered with providing such glassy electrolyte/protective barrier layers for the protection of lithium electrodes in battery cells is that the battery cell components on which the protective layer may be formed are not generally dimensionally stable, particularly where liquid electrolyte systems are used. For example, conventional polymeric electrode separator materials, such as porous polyolefins (e.g., CELGARD materials), polyacrylonitrile, etc., take up solvent and swell when contacted with liquid electrolyte. Such swelling results in elongation of the separator along its orthogonal x, y and z axes. As a result of this elongation in the x and y dimensions, a glassy protective layer formed on the surface of the separator is liable to crack and break into islands, thereby destroying its protective function.
It would desirable to be able to form separators coated with ionically conductive glassy electrolyte/protective layers and integrated lithium electrodes/separators with such glassy protective coatings as battery cell components in which the glassy protective layers would not be fractured when these components are subsequently incorporated into battery cells and brought into contact with liquid electrolytes.
Accordingly, improved methods and structures for providing protected lithium (or other active metal) electrodes for use in batteries would be desirable.